The role of isozyme studies in molecular systematics

The recent emergence of molecular techniques for obtaining evidence from DNA sequences to use in systematic studies raises the question of whether the isozyme approach is now superseded. When should the experimental taxonomist spend the limited research dollar on isozymes and when on DNA techniques? The clearest advantages of the latter are the fundamental quality of the data (being directly at the DNA level), and their potential use at all levels of the taxonomic hierarchy. The relative advantages of isozyme techniques are their lower cost, ease and rapidity. Isozymes are most suited to addressing questions at the level of populations, subspecies and species, and only of limited use at higher levels. Yet it is precisely the species level where the systematist is often seeking a variety of evidence to support taxonomic concepts. The capacity to handle a large number of samples, in a directly comparative fashion, means that isozymes are ideal for studying microevolutionary processes such as mating system, migration, local differentiation and hybridisation. These processes act on all kinds of variation, and knowing about them will assist a taxonomist's approach to other levels of evidence. Finally isozyme analysis is useful in the design of sampling strategies and the choice of samples for in-depth molecular analysis. These points are illustrated by a study of variation in Glycine canescens and polyploid origins within the G. tomentella complex, and of partial cleistogamy in G. argyrea.